This invention relates to processes for etching silicon substrates, and more particularly relates to etch techniques for producing mechanical structures, or elements, on or out of a silicon substrate.
The broad class of etch techniques known collectively as micromachining are now commonly employed to etch silicon substrates and related electronic materials to produce mechanical and electromechanical elements and systems. In addition to its well-known electrical properties, silicon has been found to exhibit superior mechanical properties. As a result, silicon is routinely employed for both electrical functionality and structural support in micromachined sensor and actuator systems.
Historically, two classes of micromachining etch techniques have been applied to silicon. In a first class, known as surface micromachining, layers of material, e.g., silicon dioxide and polycrystalline silicon (polysilicon), are applied to the surface of a silicon substrate and etched to form suspended mechanical and/or electrical elements that are attached to the substrate, or to form free elements that are removed from the substrate or held captured by other structures. The second etch technique class, known as bulk micromachining, employs processes for etching a single crystal silicon substrate itself to produce mechanical and/or electromechanical elements out of the substrate. This enables the production of suspended or free elements that are integral to the substrate.
For many applications, it is preferred that a mechanical or electromechanical element be formed of single crystal silicon rather than polysilicon. Single crystal silicon is mechanically more robust than polysilicon, and single crystal silicon exhibits superior electrical properties over that of polysilicon. As a result, an electromechanical element that is formed of single crystal silicon is in general expected to provide a range of performance advantages over a corresponding polysilicon electromechanical element.
Of particular interest is the ability to produce single crystal silicon elements that are free to move vertically and/or horizontally and that are mechanically attached to and accessible at the surface of a silicon substrate in the manner of surface-micromachined elements. This enables an electromechanical sensor and actuator configuration with processing and input/output electronics integrated locally on the same substrate. Such a configuration captures the advantages of surface micromachining, in its unobstructed access to surface elements, and of bulk micromachining, in its formation of single crystal silicon elements.
There have been proposed a wide range of processes for bulk micromachining a silicon substrate, including both wet and dry etch processes. A requirement of integration of electronics on the same substrate from which an element is to be etched places severe restrictions on the applicability of many etch processes, however; the electronics"" materials must be securely masked from or be completely impervious to the substrate etch environment. Conventional wet etch techniques are typically found to not enable this integration requirement because it is generally not possible to adequately mask substrate electronics from wet etch conditions without degrading the electronics"" material integrity.
Plasma and other dry etch processes have been proposed for overcoming the inherent limitations of wet etch conditions in bulk micromachining a silicon substrate on which electronics are integrated. Dry etch processes generally have been found to require either exotic and complicated masking and etch parameters or to not enable the degree of dimensional control that is required for many sensor and actuator applications. As a result, electromechanical, micromachined silicon elements, and particularly single crystal silicon elements, cannot be routinely fabricated with integrated electronics for addressing many sensor and actuator applications.
The invention enables the production of silicon structures using bulk micromachining plasma etch techniques. The invention specifically provides plasma etch processes that can be employed to etch both vertical trenches that define the lateral geometry of a silicon element as well as horizontal release areas under the silicon element to thereby form a suspended and mechanically moveable structure that can be employed as, e.g., an electromechanical actuator or sensor.
In the etch method of the invention, first a substrate configuration is provided that includes a silicon layer having a first face and a thickness corresponding to a specified thickness of the silicon element that is to be formed. The configuration further includes a layer of an electrically-insulating material located below and adjacent to the silicon layer. A substantially vertical trench is etched from the first face in the silicon layer to a depth that exposes the insulating layer. Then the trench in the silicon layer is exposed to a gaseous environment that is reactive with silicon, to substantially lateral etch the silicon layer preferentially at the depth of the insulating layer along a surface of the insulating layer. This lateral etch is continued for a duration that results in release of a silicon element over the insulating layer.
In accordance with the invention, this release of a silicon element can be complete, in that the element is fully separate and free from the silicon layer and thus can be completely removed from the silicon layer; or can be partial, in that the element is suspended over the insulating layer and can move in at least one direction, but is anchored to the silicon layer at at least one point. For example, the process of the invention enables the fabrication of a silicon doubly-supported or cantilever beam, or a pair of interdigitated fingers, suspended over the insulating layer.
The electrically-insulating material can be provided as, e.g., silicon dioxide that is grown, a deposited layer of silicon nitride, a spin-applied polymer, or other suitable material. The substrate configuration can be provided by, e.g., forming the electrically-insulating layer on a first silicon substrate; bonding a second silicon substrate to the formed insulating layer; and then thinning the second silicon substrate to the specified silicon element thickness. Alternatively, there can be formed a polysilicon layer of the specified silicon element thickness on the insulating layer.
In preferred embodiments, the reactive gaseous environment is a plasma environment characterized as being an anisotropic silicon etchant. In one example, the reactive gaseous environment is provided as alternating time intervals of a first environment comprising SF6 and a second environment comprising C4F8.
In accordance with the invention, it is contemplated that the lateral edges of the silicon element be photolithographically defined; these edges to be formed by the vertical trench etch in the silicon layer. In one example the photolithographic definition of the silicon element""s lateral edges is a photolithographic definition of a lateral anchor from the silicon element to the silicon layer. Here the step of reactive gaseous environment exposure of the silicon layer is carried out until the silicon element is vertically suspended over the insulating layer but laterally anchored to the silicon layer. In a further example, photolithographic definition of the silicon element""s lateral edges is a photolithographic definition of a trench that completely circumscribes the silicon element. In this case the step of reactive gaseous environment exposure of the silicon layer is carried out until the silicon element is completely released from the silicon layer.
One particular advantage of the etch process of the invention is the ability to integrate microfabricated electronics on the substrate configuration. In one example process, electronics are fabricated on the silicon layer, and then electronics are coated with a removable protective coating, prior to the trench exposure to the reactive gaseous environment. The protective coating is removed from the electronics after production of the silicon element. In one example process, an electrical interconnection is formed between the silicon element and the electronics.
To enable electrical insulation of the silicon element, it can be coated with an electrically insulating film. To provide the ability to make a back side electrical contact to the silicon layer, there is provided a process in which a via is etched from a back side of the substrate configuration through the insulating layer to expose the silicon layer. An electrically conducting layer is then formed in the via.
In another aspect, the invention provides a process for etching an angled trench in a silicon layer. In this process, a substrate configuration is provided, including a first silicon layer having at least one trench etched through the first silicon layer, and layer of an electrically-insulating material located below and adjacent to the silicon layer. The insulating layer includes an aperture that is located at a non-central location with respect to the trench and that corresponds to a prespecified trench angle. The configuration further includes a second silicon layer, in which the angled trench is to be formed, below and adjacent to the insulating layer. The trench in the first silicon layer is exposed to a gaseous environment that is reactive with silicon to etch that region of the second silicon layer that is exposed by the aperture in the insulating layer to a selected angled trench depth and prespecified trench angle.
Once the one or more angled trenches are formed in the second silicon layer, the first silicon layer and the insulating layer can be removed. This results in a substrate having angled trenches at its face.
As with the process described above, the electrically-insulating material can be provided as silicon dioxide, silicon nitride, a polymer, or other suitable material.
The substrate configuration for etching the angled trenches can be provided by a wide range of processes. For example the electrically-insulating layer can be on a first silicon substrate and then patterned to produce an aperture in the insulating layer. A second silicon substrate can then be bonded to the insulating layer. At least one trench is then etched in the second silicon substrate.
A principle advantage of silicon element formation method of the invention is that lateral geometric definition of structures, micromachining release, and dielectric passivation of a released single crystal silicon structure can all be accomplished in one piece of fabrication equipment, using a sequence of all-dry processes. Both the equipment and dry process sequence are completely compatible with conventional circuit fabrication such as CMOS and bipolar processing. This enables fabrication of mechanical and electrical structures on a common fabrication substrate and with a common fabrication facility. The angled trench etch method of the invention overcomes the severe constraints typically associated with angled etch techniques, and provides a wide range of design options for microelectromechanical structure features.
Other applications, features, and advantages of the invention will be apparent from the following description and associated drawings, and from the claims.